The present invention relates to a magnetic recording medium which is suitable for high density recording. In particular, the present invention relates to a magnetic recording medium which makes it possible to record bit information in an extremely minute area of a magnetic layer. The present invention also relates to a method for producing the magnetic recording medium and a magnetic recording apparatus.
Recent development of the advanced information society is remarkable. The multimedia technology, with which various types of information can be dealt with, is quickly popularized. A magnetic recording apparatus, which is installed, for example, to a computer, is known as one of those based on the multimedia technology. At present, the development of the magnetic recording apparatus is advanced along with a course to realize a small-sized apparatus while improving the recording density.
In order to realize high density recording with the magnetic recording apparatus, it is demanded, for example, that (1) the distance between a magnetic disk and a magnetic head is narrowed, (2) the coercive force of a magnetic recording medium is increased, (3) the speed of the signal processing process is increased, and (4) a medium, which suffers from less thermal fluctuation, is developed.
The magnetic recording medium has a magnetic layer in which magnetic particles or magnetic grains are aggregated on a substrate. Information is recorded thereon by magnetizing a certain group of several magnetic grains in an identical direction by the aid of a magnetic head. Therefore, in order to realize the high density recording, it is necessary to decrease the minimum area which may be magnetized in the identical direction at once in the magnetic layer, i.e., the unit area in which the inversion of magnetization may occur, in addition to the increase in coercive force of the magnetic layer. In order to decrease the unit area of inversion of magnetization, it is necessary that individual magnetic grains are allowed to have a fine and minute size, or it is necessary to decrease the number of magnetic grains for constructing the unit of inversion of magnetization. For example, in order to achieve a recording density above 40 Gbits/inch2 (6.20 Gbits/cm2), it is necessary that the diameter of the magnetic grain is suppressed to be not more than 10 nm. Further, it is also necessary to make countermeasures in order to decrease the dispersion of the grain diameter when the magnetic grain is allowed to have a fine and minute size, and decrease the thermal fluctuation. As a trial to realize the demands as described above, it has been suggested that a seed film is provided between a substrate and a magnetic layer, as disclosed, for example, in U.S. Pat. No. 4,652,499.
However, the method, in which the magnetic layer is provided on the substrate with the seed film intervening therebetween as described above, has had a limit to control the magnetic grain diameter and the distribution thereof in the magnetic layer. For example, even when the material for the seed film, the film formation condition, the structure of the seed film, and other factors were adjusted in order to obtain magnetic grains having a grain diameter of about 10 nm in the magnetic layer, the grain diameter distribution was broad, in which considerable amounts of grains having a size coarsely increased to several tens nm and grains inversely having a size finely decreased to about a half of 10 nm were present in a mixed manner. As for such magnetic grains, magnetic grains having a grain diameter larger than the average cause the increase in noise upon recording/reproduction. On the other hand, magnetic grains having a grain diameter smaller than the average cause the increase in thermal fluctuation upon recording/reproduction. As a result of the presence of the magnetic grains having a variety of sizes in a mixed manner, the boundary line between an area in which the inversion of magnetization occurs and an area in which the inversion of magnetization does not occur provides a coarse zigzag pattern as a whole. This fact was also a factor to increase the noise. Further, the inversion of magnetization hitherto occurred in a unit composed of a number of 5 to 10 individuals of magnetic grains in the magnetic layer of the conventional magnetic recording medium.
As for the spacing distance between the magnetic head and the magnetic layer of the magnetic recording medium for the high density recording, it is investigated that the spacing distance is narrowed to be not more than 15 nm. In general, scratches and rough irregularities exist on the substrate surface. For this reason, rough irregularities originating from the substrate have hitherto appeared on the surface of the magnetic recording medium prepared by stacking a film on the substrate. If the distance between the magnetic recording medium and the magnetic head is narrowed, it is impossible for the magnetic head to stably fly due to the rough irregularities as described above, resulting in the occurrence of the following problems. That is, the recording and reproducing characteristics are deteriorated, and the magnetic head collides with the magnetic recording medium to cause damages of the both. Therefore, it is demanded to realize a technique for forming a flat film without being affected by the surface roughness of the substrate.
On the other hand, as the spacing distance between the magnetic head and the magnetic layer is narrowed, it is more necessary to protect the magnetic layer from the shock exerted by the magnetic head and the environment of use. Therefore, it is required to form a protective film for protecting the magnetic film so that the protective film is more uniform without causing any deficiency. However, in order to realize the spacing distance of not more than 15 nm between the magnetic head and the magnetic layer, it is necessary that the protective film to be formed on the magnetic layer has a film thickness of not more than 5 nm. Even when it is intended to form a carbon protective film with a thickness of not more than 5 nm by using the conventional DC sputtering method or the magnetron sputtering method, it has been impossible to completely cover the surface of the magnetic recording medium with the protective film, because the carbon protective film is formed only in an island form, or any defect such as hollow hole and crack occurs in the protective film. If the surface of the magnetic recording medium is not completely covered with the protective film, then any corrosion occurs in the magnetic layer, and the magnetic layer suffers from any physical damage due to the head crash or the like.
In order to allow the magnetic head to fly over the magnetic disk, it is necessary to provide a texture provided with a concave/convex structure on the surface. However, it has not been easy to control the concave/convex structure to have an appropriate size.
Japanese Patent No. 2704957 discloses a magnetic recording medium having a keeper layer. The keeper layer is an auxiliary film having soft magnetization. The keeper layer is arranged so that it makes tight contact with the surface of a magnetic layer (recording layer) for performing recording. When the magnetic layer is in a recording magnetization state, a portion of the keeper layer, which contacts with a recording magnetization portion of the magnetic layer, is magnetized in a direction opposite to the magnetic layer, because the keeper layer has the soft magnetization. An annular magnetic path is formed by the recording magnetization portion of the magnetic layer and the portion of magnetization in the opposite direction of the keeper layer. Even when the film thickness of the magnetic layer is thinned, the recording magnetization is stably maintained without being demagnetized. Further, owing to such a situation, the diamagnetic field, which acts on the magnetic layer of the recording magnetization portion, is decreased. Therefore, even when the recording density is increased by allowing the recording magnetic domain to have a fine and minute size, the influence of the diamagnetic field is mitigated, because the film thickness of the magnetic layer can be made thin. The demagnetization based thereon is also reduced. Simultaneously, the recording magnetization state is stabilized by the keeper layer. Therefore, the demagnetization, which would be otherwise caused by the thermal fluctuation in accordance with the elapse of time, can be also reduced, giving high stability of storage of recording information for a long period of time.
However, when the keeper layer is formed, a problem arises such that a magnetic resistance effect (MR) magnetic head, which is widely used for the present hard disk apparatus for recording and reproduction, cannot be used as it is, because of the following reason. As clarified from the role of the keeper layer as described above, the keeper layer acts as a type of shield on the magnetic layer. Therefore, the presence of the keeper layer brings about an obstacle for the reproducing operation in which the leak magnetic field is read with the magnetic head and for the recording operation in which the magnetization of the magnetic layer is inverted with the magnetic head. That is, the recording magnetic field is obstructed by the leak magnetic field of the keeper layer during the recording. Therefore, the effective recording magnetic field is lowered. During the reproduction, an arrangement is made, in which the magnetic pole of the keeper layer counteracts the magnetic pole of the magnetic layer. For this reason, the leak magnetic field from the area of inversion of magnetization of the magnetic layer is decreased, and the reproducing sensitivity is lowered. Therefore, even when the reproducing element of the magnetic head is operated in this state, the reproduction output is small. In order to deal with the problem caused upon the recording and reproduction, the following method is known. That is, a DC bias current is allowed to flow through the magnetic head, and a DC magnetic field is applied to an area disposed just under the magnetic head, in order to eliminate the action of the keeper layer only when the recording operation and the reproducing operation are performed. With the DC magnetic field, the keeper layer of the area, on which the DC magnetic field is exerted, is magnetically saturated so that a so-called window may be effectively bored through the shield. On the other hand, as for the reproducing element of the magnetic head, it is desirable to use a magnetic resistance effect (MR) element having a high reproducing sensitivity or a giant magnetic resistance effect (GMR) element. However, such an element has no function to apply the DC magnetic field as described above. Therefore, it is impossible to perform reproduction on the magnetic recording medium provided with the keeper layer by using an ordinary MR element or an ordinary GMR element as it is.
The perpendicular magnetic recording system attracts attention as a recording system for realizing the high density recording for the magnetic recording medium. The perpendicular magnetic recording system uses a magnetic recording medium (hereinafter referred to as xe2x80x9cperpendicular magnetic recording mediumxe2x80x9d) including, as a recording layer, a magnetic layer in which the magnetization-prompt direction is perpendicular to the disk surface. The perpendicular magnetic recording of this type does not involve such a problem as caused in the in-plane magnetic recording that the magnetic field, which is generated from the boundary between magnetic domains having different magnetization directions, inhibits the formation of the minute magnetic domain. Therefore, it is possible to thicken the film thickness of the magnetic layer of the magnetic recording medium. For this reason, in the case of the perpendicular magnetic recording medium, it is possible to form a minute recording magnetic domain on the magnetic layer in order to achieve the high density recording. The perpendicular magnetic recording medium is highly resistant to the thermal fluctuation as compared with the in-plane magnetic recording medium.
As for the perpendicular magnetic recording medium as described above, a perpendicular magnetic recording medium of the monolayer type provided with only one layer of magnetic layer (recording layer) has been investigated, in which the magnetic layer of the in-plane magnetization of the in-plane magnetic recording medium is changed to the magnetic layer of the perpendicular magnetization. Although the monolayer type perpendicular magnetic recording medium has a simple structure, it involves the following problem. That is, the leak magnetic field generated from the medium is small as compared with the in-plane magnetic recording medium, and the reproduction output is small. In order to solve this problem, a two-layered type perpendicular magnetic recording medium has been suggested, in which an in-plane magnetizable layer is formed between a substrate and a magnetic layer. In the case of the two-layered type perpendicular magnetic recording medium, the magnetic flux, which is generated on the side of the substrate of the magnetic layer, passes through the in-plane magnetizable layer, and thus the magnetic path is formed. Accordingly, the leak magnetic field, which is generated on the side opposite to the substrate of the magnetic layer, is increased. Therefore, when the leak magnetic field from the magnetic layer is detected by using the reproducing head, the reproduction output is increased.
However, in the case of the two-layered type perpendicular magnetic recording medium, the magnetic flux to cause any noise, which originates from any confused magnetic domain in the area of inversion of magnetization of the magnetic layer, also passes through the in-plane magnetizable layer. Therefore, in the case of the two-layered type perpendicular magnetic recording medium, not only the reproduced signal but also the noise are increased. As a result, the signal to noise ratio (S/N) is equivalent to that of the monolayer type perpendicular magnetic recording medium. Therefore, as for the two-layered type perpendicular magnetic recording medium, it has been necessary to decrease the noise from the viewpoint of S/N.
The present invention has been achieved in order to solve the problems involved in the conventional technique as described above, a first object of which is to provide a magnetic recording medium having a magnetic layer composed of magnetic particles or magnetic grains which are allowed to have a fine and minute size and in which the dispersion of grain diameter is reduced, and to provide a magnetic recording apparatus installed with the magnetic recording medium.
A second object of the present invention is to provide a magnetic recording medium in which magnetic grains are controlled to have desired crystal orientation, and to provide a magnetic recording apparatus installed with the magnetic recording medium.
A third object of the present invention is to provide a magnetic recording medium in which the unit of inversion of magnetization is small, and to provide a magnetic recording apparatus installed with the magnetic recording medium.
A fourth object of the present invention is to provide a magnetic recording medium in which the noise is low, the thermal fluctuation is low, and the thermal demagnetization is low and which is suitable for high density recording, and to provide a magnetic recording apparatus installed with the magnetic recording medium.
A fifth object of the present invention is to provide a magnetic recording medium which is formed with a texture having a desired concave/convex structure, and to provide a magnetic recording apparatus installed with the magnetic recording medium.
A sixth object of the present invention is to provide a method for producing a magnetic recording medium provided with a protective layer composed of a super thin film to cover, with a uniform film thickness, a surface of a magnetic layer of the magnetic recording medium.
A seventh object of the present invention is to provide a magnetic recording medium and a magnetic recording apparatus suitable for high density recording, in which an MR element or a GMR element having a high reproducing sensitivity can be used for reproduction, although a layer, which plays a role of a keeper layer to mitigate recording demagnetization which would be otherwise caused by the high density recording, is provided.
An eighth object of the present invention is to provide a perpendicular magnetic recording medium and a magnetic recording apparatus in which the noise is reduced, and information can be reproduced with high S/N.
A ninth object of the present invention is to provide a super high density magnetic recording medium having a surface recording density exceeding 40 Gbits/inch2, and to provide a magnetic recording apparatus installed with the magnetic recording medium.
According to a first aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
an underlying layer which is formed on the substrate, and
a magnetic layer which is formed on the underlying layer and on which information is recorded, wherein:
the underlying layer is composed of crystal grains substantially formed of magnesium oxide, and a crystal grain boundary or boundaries containing at least one oxide selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide.
The present inventors have disclosed, in U.S. patent application Ser. No. 09/478,377 (corresponding to Japanese Patent Application No. 11-1667), a magnetic recording medium comprising a non-magnetic substrate; an inorganic compound film including crystalline first oxide composed of at least one selected from cobalt oxide, chromium oxide, iron oxide, and nickel oxide, and second oxide composed of at least one selected from silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide, in which the second oxide exists at a grain boundary of crystal grains of the first oxide; and a magnetic film formed on the inorganic compound film. In the magnetic recording medium, the crystal grains of the first oxide, which constitute the inorganic compound film, have a honeycomb structure. Magnetic particles or magnetic grains of the magnetic layer, which are formed on the inorganic compound film, are epitaxially grown from the crystal grains of the first oxide. Therefore, the magnetic grains of the magnetic layer also have a honeycomb structure. Accordingly, the crystal grains of the magnetic film are allowed to have a fine and minute size, and it is possible to uniformize the grain diameter. Thus, the magnetic recording medium is realized, in which the noise is low and the thermal fluctuation is reduced.
However, according to an experiment performed by the present inventors, when the inorganic compound film is formed on the substrate of the magnetic recording medium described above, an initial growth layer, which is an aggregate or cluster of microcrystals having no regular structure, is initially produced. It has been revealed that the inorganic compound film has to be grown so that it has a certain degree of film thickness, for example, a film thickness of not less than 30 nm until the regular honeycomb structure appears on the inorganic compound film, because of the presence of the initial growth layer. In the present invention, it has been revealed that the occurrence of the initial growth layer portion can be suppressed owing to the use of magnesium oxide as the material for the underlying layer corresponding to the inorganic compound film, especially as the material for constructing the crystal grains, and a satisfactory honeycomb structure can be formed from the initial stage of film formation. Accordingly, the thickness of the underlying layer as well as the thickness of the magnetic recording medium can be made thin. It is possible to shorten the film formation step, and it is possible to decrease the production cost.
When the first oxide, which is disclosed in U.S. patent application Ser. No. 09/478,377, is used for the crystal grains of the underlying layer, the standard deviation a of the crystal grain diameter distribution is not more than 10% of the average grain diameter. On the contrary, when magnesium oxide is used for the crystal grains of the underlying layer in the present invention, the standard deviation a of the crystal grain diameter distribution in the underlying layer has been successfully made to be not more than 8% of the average grain diameter. It is approved that the regularity of the honeycomb structure is high as the number of grains (hereinafter referred to as xe2x80x9cnumber of coordinated grainsxe2x80x9d) which surround one crystal grain of the underlying layer is close to 6.0. When magnesium oxide is used for the material for the crystalline matter, the number of coordinated grains, which is closer to 6.0, has been successfully obtained. That is, the following fact has been revealed. When magnesium oxide is used for the material for the crystalline matter, then the dispersion of the grain diameter of the underlying layer can be further decreased, and it is possible to improve the regularity of the honeycomb structure. Further, the magnetic grains of the magnetic layer to be formed on the underlying layer can be formed with the more uniform grain diameter and the more uniform structure. Therefore, in the magnetic recording medium of the present invention, the noise is low, the thermal fluctuation is low, and the thermal demagnetization is low. Further, the magnetic recording medium may be suitable for the high density recording.
The expression xe2x80x9ccrystal grains substantially formed of magnesium oxidexe2x80x9d used herein means the fact that the crystal grains may be constructed while containing not only magnesium oxide but also any impurity including, for example, oxide or element for constructing the oxide contained in the crystal grain boundary in an amount of about several %, generally in an amount of not more than 5%.
As shown in FIG. 14, the underlying layer may have such a structure that the shape of one crystal grain may be a regular hexagon in a plane parallel to the substrate surface, and the crystal grain may be grown upwardly in a pillar-shaped configuration in a cross section perpendicular to the substrate surface. Especially, the pillar-shaped cross section of the crystal grain is not widened as the underlying layer is grown, having a structure in which the width of the crystal grain boundary is uniform. Therefore, the aggregate of the crystal grains each of which forms a regular hexagonal cylinder forms the honeycomb structure in which the hexagonal cylinders are regularly arranged. Mathematically, the aggregate approximately has a fractal feature, and it can be expressed with the group theory as well. In the underlying layer, one crystal grain having the regular hexagonal configuration may be surrounded by 5.9 to 6.1 individuals of the grains in average.
As explained in an embodiment as described later on, it has been revealed that the grains deposited in the underlying layer and the grain boundary or boundaries therebetween are crystalline and amorphous respectively, by means of the lattice image observation based on the X-ray diffraction method. The standard deviation a of the crystal grain diameter distribution is not more than 8% of the average grain diameter. Further, the grain diameter distribution is a normal distribution. Therefore, it is approved that the regularity of the grain arrangement is extremely high. The crystal grains in the underlying layer has strong crystalline orientation. Therefore, when the magnetic layer is formed on the underlying layer having the structure as described above, for example, it is possible to grow the ferromagnetic magnetic grains having crystalline orientation from the crystal grain portion of the honeycomb structure. On the other hand, it is possible to grow the non-magnetic boundary portion from the crystal grain boundary of the honeycomb structure.
It is preferable that the film thickness of the underlying layer is 3 nm to 50 nm. If the film thickness of the underlying layer is less than 3 nm, it is difficult to stably form the film because of circumstances of a film-forming machine. If the film thickness exceeds 50 nm, then the thickness of the entire underlying layer is increased, and it takes a long period of time to form the film. It is desirable that the spacing distance of the crystal grains (width of the crystal grain boundary) is 0.5 nm to 2 nm, because the honeycomb structure is obtained in a stable manner, and it is possible to sufficiently suppress the magnetic interaction between the magnetic grains. The spacing distance between the crystal grains can be regulated by controlling the concentration of the oxide of inorganic compound existing in the crystal boundary and the composition ratio with respect to magnesium oxide.
It is preferable that the underlying layer is formed by means of the ECR sputtering method which utilizes the resonance discharge based on the use of the microwave as described later on. In the sputtering method, the kinetic energy of the target particle can be uniformized depending on the way of application of the bias voltage, and it is possible to control the energy more precisely. Especially, when the underlying layer is formed by using the ECR sputtering method, the film, which has the desired crystalline orientation and the satisfactory honeycomb structure, is obtained without requiring any complicated sputtering condition.
The magnetic layer, which is formed on the underlying layer, has a similar honeycomb structure reflecting or replicating the structure of the underlying layer. The magnetic grains in the magnetic layer are epitaxially grown in a continuous manner from the top of the crystal grains in the underlying layer. Therefore, when the honeycomb structure of the underlying layer is appropriately adjusted, it is possible to grow the magnetic grains having the desired grain diameter and the desired crystalline orientation thereon. That is, the underlying layer serves to control the grain diameter, the grain diameter distribution, and the crystalline orientation of the magnetic layer. The structure, the orientation, the crystal grain diameter, and other factors of the underlying layer can be controlled, for example, by selecting the concentration (composition) of the crystal grain boundary substance and magnesium oxide for forming the crystal grains, selecting the material for the crystal grain boundary, and selecting the film formation condition.
As for the magnetic layer, the magnetic grains of the magnetic layer can be grown from the crystal grains of the honeycomb structure of the underlying layer. On the other hand, the non-magnetic boundary or boundaries can be grown from the boundary or boundaries of the honeycomb structure of the underlying layer. Therefore, it is possible to provide the structure in which the magnetic grains are magnetically separated from each other. Accordingly, the unit of inversion of magnetization upon recording and reproduction can be reduced, for example, to be 2 or 3 individuals of magnetic grains. It is possible to realize the super high density recording. Further, it is possible to avoid the formation of any zigzag pattern of the boundary between the adjoining recording magnetic domains in the magnetic film, and thus it is possible reduce the noise.
Conventionally, in order to reduce the magnetic interaction between the magnetic grains, any non-magnetic element has been subjected to segregation in the vicinity of the crystal grain boundary in the crystal grain. However, in the present invention, it is possible to grow the non-magnetic portion in the magnetic layer corresponding to the crystal grain boundary which surrounds the regular hexagonal crystal grains in the underlying layer. In this case, the distance between the crystal grains in the underlying layer is controlled to be 0.5 nm to 2 nm, and the magnetic layer is epitaxially grown while reflecting this structure. Thus, it is possible to provide the non-magnetic portion having such a spacing distance in the magnetic layer. The epitaxially grown magnetic grain portion is ferromagnetic, and it has crystalline orientation suitable for the high density recording. On the other hand, the grain boundary, which surrounds the magnetic grain, resides in random orientation even when it is amorphous or crystalline. Therefore, the grain boundary exhibits the non-magnetic or the magnetization different from that of the magnetic grain portion, making it possible to allow the magnetic grains to be magnetically independent from each other. Accordingly, the size of the magnetic domain of the magnetic recording medium can be decreased to be fine and minute up to the magnetic grain size.
It is desirable for the magnetic layer to use an alloy principally containing cobalt and further containing at least two elements selected from the group consisting of chromium, platinum, tantalum, niobium, titanium, and silicon. For example, it is possible to use a film of CoCrPt or CoCrPtTa. The magnetic grain in the magnetic layer is composed of a cobalt alloy, and it may be composed of a crystalline material. The boundary between the magnetic grains may contain at least one element selected from the group consisting of chromium, tantalum, niobium, titanium, and silicon, and it may be composed of a polycrystalline material. The magnetic layer may be a multilayered film such as Co/Pt.
It is also preferable for the magnetic layer to use a magnetic film having a granular structure composed of two phases of a crystalline phase and an amorphous phase. In this case, the crystalline phase principally contains cobalt, and it further contains at least one element selected from the group consisting of neodymium, praseodymium, yttrium, lanthanum, samarium, gadolinium, terbium, dysprosium, holmium, platinum, and palladium. As for the amorphous phase, a phase of at least one compound selected from silicon oxide, zinc oxide, tantalum oxide, and aluminum oxide may exist to surround the crystal grains. For example, Coxe2x80x94SiO2 may beused. When the magnetic layer is formed as a film, cobalt grains may be grown as oxide corresponding to the crystal grain boundary on the crystal grains of the underlying layer formed by means of the ECR method.
According to a second aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
a first underlying layer which is formed on the substrate;
a second underlying layer which is formed on the first underlying layer; and
a magnetic layer which is formed on the second underlying layer and on which information is recorded, wherein:
the second underlying layer is composed of crystal grains substantially formed of at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide, and a crystal grain boundary containing at least one oxide selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, zinc oxide, and magnesium oxide; and
the first underlying layer serves as a layer to prevent the second underlying layer from initial growth.
The magnetic recording medium according to this aspect is provided with only the first underlying layer having a minute thickness of about several nanometers between the substrate and the second underlying layer including the crystal grain boundary and the group of crystal grains substantially formed of the oxide as described above. Thus, it is possible to substantially suppress the occurrence of the initial growth layer on the second underlying layer. Accordingly, it is possible to thin the thickness of the entire underlying layer as well as the thickness of the magnetic recording medium. It is possible to shorten the step of forming the film, and it is possible to decrease the production cost. Further, an advantage is also obtained such that the performance of tight contact between the substrate and the magnetic layer is enhanced owing to the provision of the first underlying layer.
It has been revealed that when the first underlying layer is provided between the substrate and the second underlying layer, the second underlying layer grows while reflecting the crystal structure of the first underlying layer and/or the morphology of the surface of the first underlying layer. For this reason, if the second underlying layer is grown on the substrate without providing the first underlying layer, the standard deviation a of the crystal grain diameter distribution of the second underlying layer is not more than 10% of the average grain diameter. On the contrary, in the case of the present invention, the standard deviation "sgr" of the crystal grain diameter distribution of the second underlying layer has been successfully not more than 8% of the average grain diameter. It is approved that the regularity of the honeycomb structure is high as the number of grains (hereinafter referred to as xe2x80x9cnumber of coordinated grainsxe2x80x9d) which surround one crystal grain of the second underlying layer is close to 6.0. When the first underlying layer is provided, the number of coordinated grains, which is closer to 6.0, has been successfully obtained. That is, the following fact has been revealed. The grain size distribution and the number of coordinated grains can be controlled by forming the first underlying layer on the substrate. Further, the magnetic grains of the magnetic layer, which are formed on the second underlying layer, can be also formed to have a more uniform grain size and a more uniform structure.
The phrase xe2x80x9ccrystal grains substantially formed of at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxidexe2x80x9d herein means the fact that the crystal grains may be constructed while containing not only the at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide but also any impurity including, for example, oxide or element for constructing the oxide contained in the crystal grain boundary in an amount of about several %.
In the present invention, an amorphous film or a crystalline film may be used as the first underlying layer. When the amorphous film is used, those usable for the amorphous film include:
(1) a metal selected from the group consisting of hafnium, titanium, tantalum, niobium, zirconium, tungsten, molybdenum, and an alloy containing at least one element of them;
(2) a cobalt alloy principally composed of cobalt and containing at least one element selected from the group consisting of titanium, tantalum, niobium, zirconium, and chromium; or
(3) at least one inorganic compound selected from the group consisting of silicon nitride, silicon oxide, and aluminum oxide. When the inorganic compound is used, it is allowable to further contain at least one metal selected from the group consisting of hafnium, titanium, tantalum, niobium, zirconium, chromium, and aluminum. When the material as described above is used, the second underlying layer can be epitaxially grown from the top of the first underlying layer more appropriately without substantially growing the initial growth layer as an aggregate of microcrystals.
On the other hand, when the crystalline film is used for the first underlying layer, the crystalline film may be composed of at least one selected from the group consisting of chromium, chromium alloy, vanadium, and vanadium alloy. In this case, the alloy may contain at least one element selected from the group consisting of titanium, tantalum, aluminum, nickel, vanadium, and zirconium. When the element as described above is added, the lattice constant of crystalline chromium or vanadium can be controlled to precisely control the crystalline structure of the second underlying layer to be formed on the first underlying layer.
When the first underlying layer is the crystalline film, it is most preferable to use the hcp (Hexagonal Closest Packing) or bcc (Body-Centered Cubic) structure, because of the following reason. That is, such a structure is a structure which is the same as or similar to the crystal structure of the magnetic layer. Therefore, it is possible to facilitate the epitaxial growth of the magnetic grains of the magnetic layer from the top of the crystal grains of the underlying layer. According to the knowledge of the present inventors, it has been revealed that the second underlying layer is grown while reflecting the crystal structure of the first underlying layer and/or the morphology of the surface of the first underlying layer. Therefore, it is preferable that the crystal structure of the first underlying layer is appropriately selected while considering the crystal structure of the second underlying layer intended to be formed thereon.
In the magnetic recording medium of the present invention, the second underlying layer contains, as the crystal grains, at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide. The grain boundary, which surrounds the crystal grains, is composed of at least one oxide selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide.
As shown in FIG. 3, the second underlying layer may have the following structure. That is, the shape of one crystal grain is a regular hexagon as viewed in a plane parallel to the substrate surface. The crystal grains are grown upwardly in a pillar-shaped configuration as viewed in a cross section perpendicular to the substrate surface of the second underlying layer. Especially, the pillar-shaped cross section of the crystal grain is not widened in a sector form even when the second underlying layer is grown, giving such a structure that the width of the grain boundary is uniform. Therefore, the aggregate of the crystal grains, in which one crystal grain forms a regular hexagonal cylinder, forms a honeycomb structure in which the hexagonal cylinders are regularly arranged. In the same manner as in the underlying layer in the first aspect of the present invention, the honeycomb structure mathematically has a fractal feature although in an approximate manner, and it can be expressed with the group theory. One crystal grain having the regular hexagonal configuration may be surrounded by 5.8 to 6.2 individuals of the grains in average.
As explained in embodiments as described later on, according to the analysis based on the X-ray diffraction method, the grains deposited in the second underlying layer and the grain boundary thereof are crystalline and amorphous respectively. The standard deviation a of the crystal grain diameter distribution is not more than 8% of the average grain diameter. Further, the grain diameter distribution is a normal distribution. Therefore, the regularity of the grain arrangement is extremely high. Further, the crystal grains in the second underlying layer have strong crystalline orientation. Therefore, when the magnetic layer is formed on the second underlying layer having the structure as described above, for example, the magnetic grains, which are ferromagnetic and subjected to crystalline orientation, can be grown from the crystal grain portions of the honeycomb structure on the other hand, the non-magnetic boundary portion can be grown from the crystal grain boundary of the honeycomb structure.
It is preferable that the first underlying layer has a film thickness of 2 nm to 50 nm. If the film thickness of the first underlying layer is less than 2 nm, it is impossible to expect the effect of the provision of the first underlying layer. If the film thickness of the first underlying layer exceeds 50 nm, then the thickness of the entire underlying layer is increased, and it takes a long period of time to form the film. It is preferable that the second underlying layer has a film thickness of 3 nm to 100 nm. If the film thickness of the second underlying layer is less than 3 nm, the good epitaxial growth of the magnetic layer scarcely takes place from the top of the underlying layer. If the film thickness of the second underlying layer exceeds 100 nm, then the thickness of the entire underlying layer is increased, and it takes a long period of time to form the film. It is preferable that the entire film thickness of the first and second underlying layers is 3 nm to 100 nm. It is desirable that the spacing distance between the crystal grains (width of the crystal grain boundary) is 0.5 nm to 2 nm, because such a spacing distance is sufficient to block the magnetic interaction between the magnetic grains in the magnetic layer formed on the underlying layer, the bulk density of the formed magnetic film is appropriate, and the recording density is improved.
According to a third aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
an underlying layer which is formed on the substrate;
a control layer which is formed on the underlying layer and which is formed of at least one selected from the group consisting of magnesium oxide, chromium alloy, and nickel alloy; and
a magnetic layer which is formed on the control layer and on which information is recorded, wherein:
the underlying layer is composed of crystal grains substantially formed of at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide, and a crystal grain boundary containing at least one oxide selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide. In the underlying layer, the respective crystal grains may have a hexagonal configuration, and they may be arranged in a honeycomb form.
According to en experiment performed by the present inventors, when the magnetic layer is formed on the underlying layer of the magnetic recording medium described above, the lattice constants of these layers are deviated from each other depending on the combination of materials for constructing the underlying layer and the magnetic layer. For this reason, the magnetic layer failed to be epitaxially grown on the underlying layer in a well-suited manner in some cases. Further, when the underlying layer is formed, it is difficult to effect complete phase separation for the crystal grains and the amorphous substance existing in the grain boundary thereof. The crystal grains are mixed with the amorphous substance in an amount of about 3 to 5% in some cases. For example, in the case of a CoOxe2x80x94SiO2 film having the honeycomb structure, it has been revealed by the xcexc-Auger analysis that several % of SiO2 may be contained in CoO in the crystal grain, while CoO may be contained in SiO2 of the amorphous substance. Therefore, even when an appropriate combination of materials is selected for the underlying layer and the magnetic film, the amorphous substance exists in the crystal grains in a mixed manner as described above. For this reason, the lattice constant of the crystal grain of the actually formed film is deviated from the original lattice constant which is expected to be obtained when no impurity exists. As a result, a situation has been probably caused, in which the lattice match is not obtained sufficiently between the crystal grains of the underlying layer and the magnetic grains of the magnetic layer. It has been revealed that if the deviation of crystal lattice occurs in an amount of not less than xc2x110% as represented by the difference in lattice constant, then the coercive force of the magnetic grains of the magnetic layer formed on the underlying layer is decreased, and it is impossible to obtain desired magnetic characteristics. It has been also revealed that if the discrepancy of the crystal lattice is further increased, then the honeycomb structure of the underlying layer is not reflected to the magnetic layer, the magnetic grains are not formed in the magnetic layer, and a polycrystalline structure is obtained as a whole.
In the present invention, the control layer (lattice constant control layer) for adjusting the discrepancy of the crystal lattices of the layers is provided between the underlying layer and the magnetic layer. Accordingly, the decrease in coercive force and the change of magnetic characteristic, which would be otherwise caused by the discrepancy of the crystal lattice, have been successfully suppressed in a substantial manner. When the control layer, in which the material is selected so that the discrepancy of the crystal lattice between the underlying layer and the control layer and between the control layer and the magnetic layer is decreased, for example, the discrepancy is within xc2x15% as represented by the difference in lattice constant of each of them, is provided, the magnetic grains of the magnetic layer can be epitaxially grown while reliably reflecting the honeycomb structure of the underlying layer. Therefore, the grain diameter of the magnetic grains of the magnetic layer can be made fine and minute by reflecting the crystal grain diameter of the underlying layer, and the magnetic grains can be surrounded by the non-magnetic boundary portion of the magnetic layer corresponding to the crystal grain boundary of the underlying layer. Accordingly, it is possible to reduce the magnetic interaction between the magnetic grains. Thus, it is possible to produce the magnetic recording medium which is suitable for the high density recording.
It is preferable to use at least one selected from the group consisting of magnesium oxide, chromium alloy, and nickel alloy for the control layer of the magnetic recording medium of the present invention. Especially, it is preferable that the control layer is formed of chromium-titanium, chromium-tungsten, magnesium oxide, or chromium-ruthenium. In the present invention, it is preferable to use, for the chromium alloy or the nickel alloy, a material containing at least one element selected from the group consisting of chromium, titanium, tantalum, vanadium, ruthenium, tungsten, molybdenum, niobium, nickel, zirconium, and aluminum, other than chromium or nickel as the base element.
It is most preferable to adopt the bcc structure or the B2 structure for the control layer. The structure is closely similar to the crystal structure of the magnetic layer to be used for the magnetic recording medium. Therefore, the lattice match is achieved between the control layer and the magnetic layer. The magnetic layer can be epitaxially grown from the control layer with ease. It is preferable to appropriately select the composition of the control layer while considering the compositions of the underlying layer and the magnetic layer so that the lattice constant of the crystal lattice of the control layer simultaneously has an approximately intermediate value between those of the underlying layer and the magnetic layer. By doing so, even when the crystal lattice of the underlying layer is different from that of the magnetic layer, it is possible to mitigate the difference by the aid of the control layer.
When the control layer is formed, it is preferable that the control layer is epitaxially grown from the underlying layer. As for the control layer, the crystalline portion is epitaxially grown from the crystal grain portion of the underlying layer, and the crystal structure or the polycrystal, which is different from the crystal grain portion, is grown from the amorphous crystal grain boundary of the underlying layer. Further, when the magnetic layer is continuously grown from the control layer, the lattice deviation between the control layer and the magnetic layer can be decreased by appropriately selecting the structure and the composition of the control layer. Therefore, the epitaxial crystal growth is facilitated, and thus an effect is obtained such that the growth of the magnetic layer is facilitated. The structure of the magnetic layer formed as described above reflects the honeycomb structure of the underlying layer. The magnetic grain diameter and the grain diameter distribution of the magnetic layer can be made substantially equal to the crystal grain diameter and the grain diameter distribution of the underlying layer. Further, the control layer also has such an effect that the adhesion force between the substrate and the magnetic layer is improved.
As described above, the underlying layer has the honeycomb structure such that the shape of one crystal grain is a regular hexagon in the plane parallel to the substrate surface of the underlying layer, and the crystal grain is grown upwardly in a pillar-shaped configuration in the plane perpendicular to the substrate surface. The magnetic layer, which is formed on the underlying layer, has a similar honeycomb structure which reflects the structure of the underlying layer. Further, the magnetic grains in the magnetic layer are epitaxially grown in the continuous manner from the top of the crystal grains in the underlying layer by the aid of the crystal grains in the control layer. Therefore, the magnetic grains, which have a desired grain diameter and crystalline orientation, can be grown in the magnetic layer to be formed on the underlying layer with the control layer intervening therebetween, by appropriately adjusting the honeycomb structure of the underlying layer.
That is, the underlying layer serves to reduce the magnetic interaction between the magnetic grains by controlling the magnetic grain diameter, the magnetic grain diameter distribution, and the orientation of the magnetic layer to be formed on the underlying layer with the control layer intervening therebetween, and growing the non-magnetic boundary portion from the crystal grain boundary having the uniform width. On the other hand, the control layer has the following effect. That is, the control layer reliably reflects the honeycomb structure of the underlying layer to the magnetic layer to facilitate the epitaxial growth by ensuring the lattice match between the crystal grains of the underlying layer and the magnetic grains of the magnetic layer. Thus, the control layer avoids the decrease in coercive force of the magnetic layer and the change of magnetic characteristics.
It is preferable that the underlying layer and the control layer are formed in accordance with the ECR sputtering method which utilizes the resonance discharge based on the used of the microwave. It is preferable that the film thickness of the underlying layer is 2 nm to 50 nm. If the film thickness of the underlying layer is less than 2 nm, it is difficult for the magnetic grains of the magnetic layer to cause the epitaxial growth in a well-suited manner. If the film thickness of the underlying layer exceeds 50 nm, then the thickness of the underlying layer is increased, and it takes a long period of time to form the film. It is preferable that the film thickness of the control layer is 2 nm to 10 nm. If the film thickness of the control layer is less than 2 nm, it is impossible to obtain the film which has the good crystal structure. If the film thickness of the control layer exceeds 10 nm, then the entire thickness is increased, and it takes a long period of time to form the film. Accordingly, considering the fact that the two layers are used for the underlying base for forming the magnetic layer for the magnetic recording medium, it is most preferable that the film thickness of the two layers is 5 nm to 100 nm.
It is preferable that the crystal structures of the underlying layer and the control layer and the crystal structures of the control layer and the magnetic layer are similar to one another. That is, it is preferable that any one of crystal forms of the crystal grains of the underlying layer and the control layer and any one of crystal forms of the magnetic layer (for example, the crystal structure, the crystal shape, and the crystal size) are substantially equal to one another, and the differences in lattice constant between the underlying layer and the control layer and between the control layer and the magnetic layer are within xc2x15% respectively. Accordingly, the magnetic grains of the magnetic layer can be epitaxially grown in a well-suited manner from the top of the crystal grains of the underlying layer while reflecting the honeycomb structure of the underlying layer with the crystal grains of the control layer intervening therebetween. Therefore, in the present invention, even when the difference in lattice constant between the underlying layer and the control layer is not less than xc2x110%, the uniform and fine magnetic grains can be epitaxially grown in the magnetic layer while mitigating the difference by providing the plurality of layers for adjusting the lattice plane between the underlying layer and the magnetic layer. The number of control layer is not limited to single. A plurality of control layers may be provided to disperse the difference in lattice constant between the underlying layer and the magnetic layer at boundaries between the respective layers.
In the present invention, preferred combinations of materials for constructing the stack of the underlying layer/control layer/magnetic layer include CoOxe2x80x94ZnO/Crxe2x80x94Ti alloy/Coxe2x80x94Crxe2x80x94Pt alloy, CoOxe2x80x94SiO2/MgO/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, CoOxe2x80x94SiO2/Crxe2x80x94W alloy/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, CoOxe2x80x94SiO2/MgO/Coxe2x80x94SiO2 granular type magnetic film, CoOxe2x80x94SiO2/Nixe2x80x94Al alloy/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, CoOxe2x80x94SiO2/Crxe2x80x94Ti alloy/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, CoOxe2x80x94SiO2/Nixe2x80x94Ta alloy/Coxe2x80x94Ptxe2x80x94SiO2 granular type magnetic film, CoOxe2x80x94SiO2/Nixe2x80x94Ta alloy/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, CoOxe2x80x94SiO2/Crxe2x80x94Ru alloy/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, CoOxe2x80x94SiO2/Crxe2x80x94Ru alloy/Coxe2x80x94Ptxe2x80x94SiO2 granular type magnetic film, CoOxe2x80x94SiO2/Coxe2x80x94Crxe2x80x94Zr alloy/Coxe2x80x94Ptxe2x80x94SiO2 granular type magnetic film, CoOxe2x80x94SiO2/Coxe2x80x94Crxe2x80x94Zr alloy/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, CoOxe2x80x94SiO2/Crxe2x80x94Mo alloy/Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy, and CoOxe2x80x94SiO2/Crxe2x80x94Mo alloy/Coxe2x80x94Ptxe2x80x94SiO2 granular type magnetic film. When such a combination is selected, then the structure and the grain diameter distribution of the magnetic grains of the magnetic layer can be controlled more appropriately to produce the magnetic recording medium which is suitable for the high density recording.
According to a fourth aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
an underlying layer which is formed on the substrate; and
a magnetic layer which is formed on the underlying layer and on which information is recorded, wherein:
the underlying layer is composed of crystal grains and a crystal grain boundary which surrounds the respective crystal grains, the crystal grains being arranged in a honeycomb configuration; and
the crystal grains protrude at a height of 3 to 20 nm from a surface of the underlying layer.
As for the texture of the surface desirable for the high density recording, it is desirable that the distance value, which ranges from the surface of the magnetic recording medium or the convex portion (apex) of the magnetic layer to the convex portion (apex) nearest to the foregoing convex portion, is smaller than the upper limit value of the control limit of the flying amount of the magnetic head. Further, it is desirable that the protruding amount (height) of the convex portion of the magnetic layer or the surface of the magnetic recording medium, i.e., the distance from the apex of the magnetic layer or the surface of the magnetic recording medium to the valley nearest to the apex is smaller than the upper limit value of the control limit of the flying amount of the magnetic head. In the magnetic recording medium of the present invention, as shown in FIG. 8, the underlying layer is provided with the crystal grains (12) which protrude by the height (16) of 3 to 20 nm from the surface of the underlying layer (surface of the crystal grain boundary 14). Therefore, the magnetic recording medium of the present invention satisfies the demand as described above. The underlying layer having such a structure may be formed while appropriately controlling the sputtering condition by using the ECR sputtering method. It is desirable that the crystal grain of the underlying layer is substantially formed of at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide, and the crystal grain boundary is composed of at least one oxide selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide. It is desirable that the underlying layer has a film thickness of 10 nm to 100 nm. The magnetic layer or a protective layer which may be formed thereon may have projections protruding by a height of 3 to 20 nm from a surface of the magnetic layer or a surface of the protective layer while reflecting a surface structure of the underlying layer. The projections may be used as a texture of the magnetic recording medium. Considering the size of the crystal grains, it is desirable that a distance between the adjoining projections is 10 to 30 nm. When the distance is within the range as described above, the projections also function as the texture in a well-suited manner.
According to a fourth aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
an underlying layer which is formed on the substrate; and
a magnetic layer which is formed on the underlying layer, wherein:
the underlying layer has soft magnetization, and the underlying layer is composed of crystal grains substantially formed of at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide, and a crystal grain boundary containing at least one oxide selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide.
The magnetic recording medium according to this aspect is provided with the underlying layer having the two features as described below between the substrate and the magnetic layer. The first feature of the underlying layer is that the underlying layer has the structure in which the plurality of crystal grains are surrounded by the crystal grain boundary respectively and they are regularly arranged in the honeycomb configuration as described above. In the present invention, when the magnetic layer is stacked on the underlying layer as described above, the regular honeycomb structure of the magnetic grains can be brought about for the magnetic layer by reflecting the honeycomb structure of the underlying layer.
The second feature of the underlying layer is that the underlying layer is provided with the soft magnetization. The underlying layer having the soft magnetization functions as a keeper layer as described above. Therefore, it is possible to suppress the demagnetization which would be otherwise caused by the diamagnetic field of the recording magnetization area of the magnetic layer, and it is possible to stably retain the recorded magnetization state of the magnetic recording medium. Further, it is also possible to thin the film thickness of the magnetic layer. Therefore, when the underlying layer as described above is provided, it is possible to realize the magnetic recording medium which is excellent in long term storage performance.
In order to allow the underlying layer to have the soft magnetization, the composition of the crystal grain in the underlying layer may be appropriately changed. For example, in the case of a CoOxe2x80x94SiO2 film used in an embodiment described later on, the soft magnetization can be generated by progressively deviating the composition of the crystal grain principally composed of CoO from the stoichiometric composition (Xxe2x89xa00 in the case of expression with CoO1xe2x88x92X), i.e., by allowing Co to exist in the CoO film. This can be achieved, for example, by using a sputtering gas of a mixed gas (reducing atmosphere) including Ar gas mixed with H2 in an amount of about 1%, when the underlying layer is formed by means of sputtering. The metal Co atom having magnetization is generated in CoO as described above, and thus the soft magnetization is successfully brought about for the underlying layer. As for the soft magnetization of the underlying layer, it is preferable that the underlying layer has a coercive force of 0.05 (Oe) to 10 (Oe) (about 3.95 A/m to about 790 A/m) and a relative permeability of 500 to 10000, in order that the underlying layer functions as the keeper layer.
When the keeper layer is provided for the magnetic recording medium, problems arise concerning the recording sensitivity and the reproducing sensitivity as described above. In the present invention, taking notice of the fact that the underlying layer having the soft magnetization has a relatively low Curie temperature, the area, on which information is to be recorded, is irradiated with a convergent light beam when the magnetic recording medium is subjected to recording (or reproduction) to apply a recording magnetic field to the area in a state in which the temperature of the area is locally raised. In this case, the coercive force is lowered in the area irradiated with the light beam due to the increase in temperature. When the temperature exceeds the Curie temperature, the magnetization of the area disappears. In this state, the magnetic recording medium is effectively equivalent to a magnetic recording medium in which the underlying layer as the keeper layer does not exist. That is, in the present invention, although the underlying layer exists, it is enough to use a small recording magnetic field in order to generate the inversion of magnetization. Also during the reproduction, a reproducing magnetic field is applied while irradiating an information-reproducing area with light. When such a reproducing method (light assist reproducing method) is used, it is possible to detect the magnetization information at a high sensitivity for the leak magnetic field from the magnetic layer without being inhibited by the keeper layer. When the recording or the reproduction is completed, and the temperature of the area in which the information is recorded or reproduced is lowered, then the coercive force is gradually increased, and the underlying layer restores the soft magnetization. At the room temperature, the direction of magnetization of the underlying layer is directed in a direction opposite to the direction of magnetization of the magnetic layer due to the leak magnetic field generated from the boundary of the area of inversion of magnetization of the magnetic layer, i.e., the area having the different direction of magnetization, and the annular magnetic path is formed through the magnetic layer and the underlying layer. Therefore, the recording magnetization state is stabilized.
According to a sixth aspect of the present invention, there is provided a magnetic recording medium comprising:
a substrate;
an underlying layer which is formed on the substrate; and
a magnetic layer which is formed on the underlying layer and which has a magnetization-prompt direction in a direction perpendicular to a substrate surface, wherein:
the underlying layer is composed of crystal grains substantially formed of at least one oxide selected from the group consisting of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide, and a crystal grain boundary containing at least one oxide selected from the group consisting of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide.
The magnetic recording medium of this aspect is provided with the underlying layer composed of the crystal grains substantially formed of the specified oxide and the crystal grain boundary containing the specified oxide for surrounding the respective crystal grains, the underlying layer being disposed between the substrate and the magnetic layer (recording layer) for recording information thereon. As for the underlying layer, it is possible to freely control the diameter and the distribution of the crystal grains of the material for constructing the magnetic layer. Therefore, it is possible to reduce the noise generated from the magnetic recording medium. In general, the noise, which is generated from the magnetic recording medium, includes a component which is generated even after the direct current demagnetization irrelevant to the recording density, and a component which is increased in accordance with the increase in recording density. It has been revealed for the perpendicular magnetic recording medium that the noise, which is generated even after the direct current demagnetization, is decreased by strengthening the perpendicular magnetic anisotropy of the magnetic layer having the perpendicular magnetization to increase the horny ratio of the magnetization curve in the vertical direction. Accordingly, investigation has been made for the component as the other noise component which is increased in accordance with the increase in recording density, in a state in which the noise, which is generated even after the direct current demagnetization, is reduced by means of the method as described above. As a result, it has been revealed that the latter noise is principally generated in the area of inversion of magnetization (boundary between adjoining recording magnetic domains).
The noise, which is generated in the area of inversion of magnetization, results from the fact that the crystal grains of the material for constructing the magnetic layer are large. That is, when the size of the crystal grain is large, then the area of inversion of magnetization is decreased in the circumferential direction of the disk-shaped recording medium, and the area of inversion of magnetization has a zigzag form. Therefore, in order to reduce the noise generated in the area of inversion of magnetization, it is desirable that the size of the crystal grain is small. However, if the crystal grain diameter is extremely small to be about several nm, the crystal grains undergo the diamagnetic field for a long period of time when the magnetic recording medium is stored for the long period of time. As a result, the magnetization is decreased due to the demagnetization action of thermal fluctuation, and the reproduction output is decreased when information is reproduced. Therefore, it is necessary that the crystal grain diameter has an appropriate size, and it is desirable that the distribution thereof is as small as possible as well. Even when the size of the crystal grain is decreased, if the magnetic interaction between the crystal grains is large, then the same state is magnetically given as the state in which large crystal grains exist. Therefore, in order to reduce the noise, it is desirable that the crystal grains are magnetically isolated.
Accordingly, in the present invention, in order to realize the state as described above, the underlying layer is provided between the substrate and the magnetic layer. The crystal grains are composed of at least one of cobalt oxide, chromium oxide, iron oxide, nickel oxide, and magnesium oxide. The crystal grain boundary is composed of at least one oxide of silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, and zinc oxide. In the underlying layer as described above, the size of the crystal grains composed of deposited cobalt oxide, chromium oxide, iron oxide, or nickel oxide is constant in accordance with the film formation condition when the underlying layer is formed as the film on the substrate. Further, the crystal grains may be arranged in a honeycomb configuration. That is, the respective deposited crystal grains have the regular hexagonal shape in a plane parallel to the substrate surface and they are pillar-shaped in a cross section perpendicular to the substrate surface of the underlying layer. The crystal grains, each of which forms the regular hexagonal cylinder, may be gathered into an aggregate to form a honeycomb structure in which the regular hexagonal cylinders are regularly arranged.
When the magnetic layer is formed on the underlying layer as described above, the honeycomb structure, which is similar to that of the underlying layer, is formed on the formed magnetic layer while reflecting the structure of the underlying layer. The crystal grains in the magnetic layer are epitaxially grown continuously from the top of the crystal grains in the underlying layer. Therefore, the magnetic grains, which have the desired grain diameter and the desired crystalline orientation, can be grown on the underlying layer by appropriately adjusting the honeycomb structure of the underlying layer. As described above, the underlying layer has the action to control the grain diameter, the grain diameter distribution, and the crystalline orientation of the magnetic layer. Therefore, it is possible to realize the fine and minute crystal grain diameter of the magnetic layer, and it is possible to eliminate the dispersion of the grain diameter. It is possible to decrease the thermal fluctuation and the noise of the magnetic recording medium resulting from the above. Further, the area of inversion of magnetization in the magnetic layer is prevented from formation of the zigzag pattern. Therefore, it is possible to reduce the noise. In order to control, for example, the structure, the orientation, and the crystal grain diameter of the underlying layer, for example, selection may be made appropriately and preferably, for example, for the concentration (composition) of the oxide for forming the crystal grains and the crystal grain boundary substance, the material for the crystal grain boundary, and the film formation condition.
In the present invention, it is desirable that the crystal grains of the underlying layer are oriented in (111) orientation, because of the following reason. That is, when the magnetic layer is formed on the underlying layer oriented in the orientation described above, the magnetic layer can be easily oriented in (00.1) orientation. The magnetic layer, which is oriented in (00.1) orientation, exhibits the perpendicular magnetization.
In the magnetic recording medium of the present invention, it is desirable that the magnetic layer is composed of, for example, an alloy principally containing Co and containing at least two elements selected from Cr, Pt, Ta, Nb, Ti, and Si. It is desirable that the alloy as described above exhibits the ferromagnetic property. The alloy as described above has a large leak magnetic field because the saturation magnetization is large, making it possible to increase the obtained reproduced signal.
It is preferable that the magnetic recording medium of the present invention is provided with a control layer in order to reinforce the orientation of the magnetic layer, the control layer being disposed between the underlying layer and the magnetic layer. The control layer may be composed of Ti or an alloy principally containing Ti. When such a control layer is provided, it is possible to give a desired magnetization-prompt direction to the magnetic layer. Therefore, it is possible to obtain better characteristics. In this case, the shape, the size, and the arrangement of the crystal grains of the control layer follow those of the underlying layer. Therefore, the shape, the size, and the arrangement of the crystal grains of the underlying layer are also inherited by the magnetic layer which is formed on the control layer.
It is preferable that the magnetic recording medium of the present invention comprises a soft magnetic layer which is provided between the substrate and the underlying layer. It is preferable that the soft magnetic layer is composed of an amorphous material. Those preferably usable for such a material include, for example, CoNbZr, FeCoZrB, and FeCoSiB. It is desirable that the material for constructing the soft magnetic layer has the following magnetic characteristics. That is, the magnetic characteristics make it possible to allow the magnetic flux to pass sufficiently and avoid any change of the recording state of the recording layer, which would be otherwise caused such that the external magnetic field is amplified by the magnetic path formed by the magnetic head and the magnetic recording medium. For example, it is desirable that the coercive force is 5 (Oe) (about 400 A/m), and the magnetic permeability is not less than 100 and not more than 10000.
According to an eighth aspect of the present invention, there is provided a magnetic recording apparatus comprising the magnetic recording medium in accordance with any one of the first to seventh aspects of the present invention.
According to a ninth aspect of the present invention, there is provided a method for producing a magnetic recording medium comprising, on a substrate, a magnetic layer for recording information thereon and a protective layer, the method comprising:
generating plasma by means of resonance absorption;
allowing the generated plasma to collide with a target so that target particles are sputtered; and
applying a bias voltage between the substrate and the target to introduce and deposit the sputtered target particles on the substrate so that at least one layer of the magnetic layer and the protective layer is formed.
In the present invention, the particles such as electrons are accelerated by means of the resonance absorption of the energy of the electromagnetic wave such as a microwave, and they collide with the gas to cause ionization of the gas. Thus, the plasma having high energy is generated. The particles, which constitute the plasma, have the high energy, and the energy of each of the particles is uniform as compared with ordinary plasma generated by electric discharge or the like. The plasma, in which the energy distribution is narrow, is obtained. The plasma collides with the target by the aid of the bias voltage to drive out the target particles. During this process, the kinetic energy of the plasma to collide with the target, and consequently the kinetic energy of the target particles driven out by the plasma can be precisely controlled by changing the bias voltage. The target particles, in which the energy is controlled as described above, are directed toward the substrate as the flow of the target particles, and they are successfully deposited on the substrate uniformly with an equivalent film thickness. When this method is used to produce the magnetic recording medium, it is possible to make control and obtain a desired value for any one the density of the thin film to be formed, the flatness of the surface, the crystalline orientation, the orientation of crystal growth, the crystal structure, and the crystal grain diameter by appropriately selecting the material and the film formation condition. Further, when this method is used to form thin films of two or more layers, it is possible to suppress the mutual substrate diffusion between the thin films. For example, when the protective layer is formed on the magnetic layer by means of this method, it is possible to avoid any damage of magnetic characteristics of the magnetic layer, which would be otherwise caused by the diffusion of the component of the magnetic grain to the protective layer. Additionally, when this technique is used, it is possible to reduce the crystalline deficiency in the thin film. Accordingly, it is possible to form a dense film, and it is possible to obtain crystals which is strongly oriented in a constant orientation.
When the electrons are excited by means of the resonance absorption, it is also possible to use an electromagnetic wave in a region other than the region of the microwave. However, it is preferable to use the microwave. Further, it is preferable to use an alternating current power source having a radio frequency (RF) or a direct current power source (DC) as a bias power source which is used to control the kinetic energy of the plasma and the target particles to have a constant value. For example, when a conductive material such as carbon is subjected to ECR sputtering, it is possible to use the DC power source on the other hand, when a nonconductive material such as silicon oxide is subjected to ECR sputtering, it is possible to use the RF power source. The selection of the use of either the DC power source or the RF power source as the bias power source is determined depending on the characteristics and the structure of the thin film intended to be obtained.
In this specification, the term xe2x80x9cresonance absorptionxe2x80x9d refers to a phenomenon in which the particle, which performs the periodic motion, absorbs the energy of the electromagnetic wave, and the amplitude in the periodic motion of the particle, i.e., the energy possessed by the particle is remarkably increased, when the angular frequency of the particle which receives the action of the external force and which performs the periodic motion at the specified angular frequency is approximately coincident with the frequency of the electromagnetic wave incoming from the outside.
When the protective layer is formed as described above, then the target may be carbon, and a mixed gas, which principally contains argon and which contains at least one of nitrogen and hydrogen, may be used as a plasma gas. When the protective film is formed by using the method of the present invention, it is possible to obtain the protective film having a uniform film thickness of 1 to 5 nm. Further, it is possible to obtain the protective film having a density of 60% of the theoretical density.
According to a tenth aspect of the present invention, there is provided a method for producing a magnetic recording medium comprising, on a substrate, an underlying layer and a magnetic layer for recording information thereon, the method comprising:
generating plasma by means of resonance absorption;
allowing the generated plasma to collide with a target so that target particles are sputtered; and
applying a bias voltage between the substrate and the target to introduce and deposit the sputtered target particles on the substrate so that the underlying layer is formed.
In the production method according to the tenth aspect of the present invention, the underlying layer is formed by using the so-called ECR sputtering method prior to the formation of the magnetic layer. When the magnetic layer is formed on the underlying layer, it is possible to more precisely control at least one of the parameters of the crystal structure of the magnetic layer, the crystalline orientation, the crystal grain diameter, the grain diameter distribution, and the density and the surface flatness of the formed film. Accordingly, it is possible to produce the magnetic recording medium which makes it possible to perform the super high density recording.
The underlying layer is composed of an amorphous portion and a crystalline portion comprising crystal grains having a uniform grain diameter, by forming the film by using the ECR sputtering method described above, making it possible to provide a structure in which the crystal grains are separated from each other by the amorphous portion (crystal grain boundary) having a uniform width. Further, when the ECR sputtering method described above is used, the crystal grains of the underlying layer can be oriented in a constant crystalline orientation. As for the structure of the underlying layer, when the ECR sputtering method described above is used, it is possible to control, for example, the crystal grain diameter, the width of the grain boundary, and the crystalline orientation by changing the material and the film formation condition. It is possible to form a desired structure. For example, in an embodiment described later on, an underlying layer has been successfully formed, in which hexagonal crystal grains are regularly arranged in a honeycomb configuration with a crystal grain boundary intervening therebetween. The number of crystal grains deposited around one crystal grain is 5.9 to 6.1. The underlying layer, which has an extremely regular honeycomb structure, has been successfully formed. The underlying layer as described above can be formed such that the crystal grains are formed with cobalt oxide, chromium oxide, magnesium oxide, iron oxide, nickel oxide, or a combination of these oxides, and the crystal grain boundary is formed with silicon oxide, aluminum oxide, titanium oxide, tantalum oxide, zinc oxide, or a combination of these oxides. It is preferable that such an underlying layer has a film thickness of 2 to 50 nm.
When the underlying layer is formed, the target particles may be sputtered in a reactive atmosphere containing oxygen (reactive ECR sputtering method). The velocity of film formation can be improved to be 2-fold to 3-fold as compared with a case in which the atmosphere is not the reactive atmosphere, for example, by using a mixed gas containing oxygen in argon as a sputtering gas.
A thin film, which has the bcc structure or the B2 structure, may be used other than the underlying layer constructed with the compound described above. Especially, it is possible for the thin film to use an inorganic compound such as magnesium oxide, chromium, nickel, chromium alloy, or nickel alloy. It is preferable that such an alloy contains solid solution of chromium, titanium, tantalum, vanadium, ruthenium, tungsten, molybdenum, vanadium, niobium, nickel, zirconium, aluminum, or a combination of these element, other than chromium or nickel as the base element. Further, it is preferable that the underlying layer is oriented in a certain orientation. It is preferable that such a thin film has a film thickness of 2 to 10 nm.
When the magnetic layer is formed, it is preferable that the magnetic layer is epitaxially grown on the underlying layer with the structure controlled as described above. As a result, the obtained magnetic layer reflects the structure of the underlying layer, and it has such a structure that the magnetic grains grown on the crystal grains of the underlying layer are uniformly separated by the non-magnetic portion grown on the crystal grain boundary of the underlying layer.
Accordingly, it is possible to reduce the magnetic interaction between the magnetic grains, and it is possible to decrease the unit of inversion of magnetization. Further, the grain diameter of the magnetic grain is equal to the crystal grain diameter of the underlying layer, making it possible to miniaturize the magnetic grain diameter and reduce the dispersion of the grain diameter. Therefore, it is possible to obtain the magnetic recording medium in which the thermal fluctuation and the thermal demagnetization are small. In an embodiment described later on, the standard deviation ("sgr") in the magnetic grain diameter distribution is not more than 8% of the average grain diameter, and thus the dispersion of the grain diameter has been successfully decreased. Further, the crystalline orientation can be made in the constant orientation by epitaxially growing the magnetic grains on the crystal grains of the underlying layer. In an embodiment described later on, the crystalline orientation of (11.0), which is preferable for the high density recording, has been successfully obtained for Co in the magnetic layer. Further, when the magnetic layer is formed as the film by means of the sputtering method based on the use of the resonance absorption described above, then it is possible to more precisely control the structure and the orientation of the magnetic layer, and the epitaxial growth of the magnetic layer from the underlying layer is facilitated. Therefore, the obtained film is preferred for the high density recording as compared with a conventional film formed by the DC sputtering method or the magnetron sputtering method.
The material for the magnetic layer is an alloy principally containing cobalt. It is preferable to contain cobalt as well as chromium, platinum, tantalum, niobium, titanium, silicon, boron, phosphorus, palladium, vanadium, terbium, gadolinium, samarium, neodymium, dysprosium, holmium, europium, or a combination of these elements. When chromium, tantalum, niobium, titanium, silicon, boron, phosphorus, or a combination of them is contained in the magnetic layer, such an element is deposited (segregated) at the grain boundary or in the vicinity of the crystal grain boundary of the crystal grains of cobalt. It is also possible to decrease the magnetic interaction between the magnetic grains by means of the segregation.
Other than the above, it is also possible for the magnetic layer to use a magnetic film having the granular structure in which crystal grains of metal exist while being surrounded by an amorphous phase. It is preferable that the crystal grains are composed of cobalt or alloy principally containing cobalt, containing neodymium, praseodymium, yttrium, lanthanum, samarium, gadolinium, terbium, dysprosium, holmium, platinum, palladium, or a combination of these elements. It is preferable that the amorphous portion, which exists to surround the metal crystal grains, is composed of silicon oxide, aluminum oxide, titanium oxide, zinc oxide, silicon nitride, or a combination of these compounds. In the case of the magnetic film having the granular structure, it is possible to reduce the magnetic interaction between the magnetic grains owing to the presence of the amorphous portion, in the same manner as in the segregation described above. Alternatively, the magnetic layer may be an artificial lattice multilayered film obtained by alternately stacking Co and Pt. Such a film may be formed by co-sputtering targets of Co and Pt.
A plurality of underlying layers may be provided, if necessary. For example, when the difference in lattice constant is large between the underlying layer and the magnetic layer, it is possible to facilitate the epitaxial growth of the magnetic layer in a well-suited manner by inserting therebetween a layer (control layer) having an intermediate lattice constant between those of the two layers. In this case, in addition to the underlying layer described above, the magnetic layer may be formed with a magnesium oxide layer or an alloy layer principally containing chromium or nickel intervening therebetween.
Further, in order to isolate the magnetic layer from the atmospheric air and protect the magnetic layer from any shock received from the magnetic head, it is possible to form a protective layer on the magnetic layer (on the side to make contact with the magnetic head) by using the ECR sputtering method described above. An upper limit exists for the film thickness of the protective layer because of the high density recording as described above. It is desirable that the film thickness of the protective layer is not more than 5 nm. When the ECR sputtering method is used, it is possible to control the kinetic energy of the target particles. Therefore, even when the film thickness is thin, it is possible to form the protective layer which is dense and which coats the magnetic layer with the uniform thickness. In order to stably form the protective layer by means of this method, it is enough to provide a film thickness of not less than 1 nm which is extremely thin as compared with the conventional film thickness of 10 nm. When the ECR sputtering method is used for a carbon film of the protective layer, the density of the carbon film is not less than 60% of the theoretical density (density obtained in a state in which carbon atoms are accumulated without any gap). It is possible to form a denser film as compared with the conventional film of 40 to 50%. Further, the hardness of the film is not less than two-fold as compared with a film formed by means of the conventional sputtering method (for example, the RF magnetron method).
It is preferable that the protective layer is composed of a carbon thin film. The protective layer may be formed in an electric discharge gas atmosphere principally containing argon. It is preferable that the protective layer is formed in a mixed gas atmosphere containing argon as well as at least one gas selected from nitrogen and hydrogen. When the film is formed by using the mixed gas, nitrogen and hydrogen are contained in the obtained thin film. Accordingly, it is possible to facilitate the dense feature of the carbon thin film as the protective layer.
As indicated by a result of measurement performed with an interatomic force electron microscope (AFM) in an embodiment as described later on, when the underlying layer is formed by means of the ECR sputtering method, it is possible to realize the flat feature of the film surface without being affected by scratches and rough irregularities on the substrate surface. On the other hand, a minute and regular concave/convex pattern, which results from the slight difference in growth speed between those grown on the crystal grain boundary and those grown on the crystal grains of the underlying layer, appears on the surface of the magnetic layer by epitaxially growing the magnetic layer on the underlying layer. Further, when the protective layer is formed on the magnetic layer by means of the ECR sputtering method, the protective layer covers the surface of the magnetic layer with the uniform film thickness. Therefore, the surface of the protective layer has a shape which reflects the regular concave/convex pattern of the magnetic layer. The minute concave/convex pattern is useful as a texture for allowing the magnetic head to stably fly over the magnetic recording medium.
According to the present invention, it is possible to produce the magnetic recording medium capable of performing the high density recording in which the surface recording density exceeds 40 Gbits/inch2.